Serotonin (5-HT) is a neurotransmitter that has been implicated in the aetiology of numerous Mental Health disorders, including depression, anxiety, social phobia, schizophrenia, obsessive-compulsive and panic disorders, migraine, and eating disorders. There are 12 5-HT receptor subtypes with similar recognition properties yet widely divergent physiological roles that belong to the same family of G Protein Coupled Receptors (GPCRs), hindering the discovery of subtype-selective drugs. Knowledge of the 3-dimensional structure of these receptors would enable the design of selective drugs. However, solving the structure at atomic resolution of membrane proteins such as GPCRs has been hampered by the difficulty to purify sufficient amounts of homogeneous and functional protein. Here, it is proposed a novel, proprietary expression system combined with a purification method to generate large amounts of high quality purified receptor, which could solve the bottleneck for structural elucidation of these important drug targets. In Phase I, the feasibility of the proposed approach will be tested for all the 12 human 5-HT GPCRs. In Phase II, this approach will be developed resulting in purified homogenous protein for each human 5- HT receptor subtype. This high quality purified material will be a highly valuable product enabling structural elucidation of these receptors. [unreadable] [unreadable]